A method of depositing structural layers during manufacture of surface-micromachined devices sometimes involves the use of an epitaxial reactor. Epitaxy is a process for production of layers of monocrystalline layers of silicon over a single crystal substrate, and for forming polycrystalline silicon layers over other substrate materials, for instance SiO2 films on silicon substrates. Epitaxial reactors may be operated with precisely controlled temperature and environmental conditions to ensure uniform deposition and chemical composition of the layer(s) being deposited on the target substrate. In addition to the precise control, use of an epitaxial reactor may permit build-up of layers on a substrate at significantly higher rates than typically found with LPCVD (Low Pressure Chemical Vapor Deposition) systems.
U.S. Pat. No. 6,318,175 discusses an approach to using epitaxial deposition to create a micromachined device such as a rotation sensor.
While the foregoing micromachining operations or similar processes may provide acceptable products for many applications, some applications may require finer width gaps between the micromachined elements on the device than can be provided by this process. Some applications may require, for instance, obtaining higher working capacitances and/or electrostatic forces between micromachined structures. While etching very narrow trenches to obtain desired narrow gaps has been attempted, these methods may require slower etch rates, may be limited in aspect ratio, and may be subject to limitations of the lithography and etching process. Similarly, germanium has been applied to produce narrow gaps, however, this process may have process compatibility limitations.
Accordingly, there is a need for a process for manufacturing devices which provides product with inter-element gaps that may be precisely defined or “tuned” to meet the device design objectives, while still maintaining satisfactory production rates.